Mechanical coupling in a multi-chip module using magnetic components

ABSTRACT

A multi-chip module (MCM) is described. This MCM includes at least two substrates that are remateably mechanically coupled by positive and negative features on facing surfaces of the substrates. These positive and negative features mate with each other. In particular, a positive feature may mate with a given pair of negative features, which includes negative features on each of the substrates. Furthermore, at least one of the negative features in the given pair may include a hard magnetic material, and the positive feature and the other negative feature in the given pair may include a soft magnetic material that provide a flux-return path to the hard magnetic material. In this way, the hard magnetic material may facilitate the remateable mechanical coupling of the substrates.

GOVERNMENT LICENSE RIGHTS

The United States Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofAgreement No. HR0011-08-9-0001 awarded by the Defense Advanced ResearchProjects Administration.

BACKGROUND

1. Field

The present disclosure generally relates to multi-chip modules (MCMs).More specifically, the present disclosure relates to an MCM thatincludes substrates that are mechanically coupled using magneticcomponents.

2. Related Art

As integrated-circuit (IC) technology continues to scale to smallercritical dimensions, it is increasingly difficult for existinginter-chip connections to provide suitable communicationcharacteristics, such as: high bandwidth, low power, reliability and lowcost. A variety of interconnect technologies have been proposed toaddress this problem, including proximity communication or PxC (forexample, using capacitive inter-chip contacts).

PxC based on capacitive inter-chip contacts provides dense inter-chipconnections, with a pitch between neighboring pads on the order of10-100 μm. However, PxC introduces additional packaging challenges. Inparticular, in order to achieve high-bandwidth/high-data-rate inter-chipcommunication, PxC typically requires mechanical alignment betweenfacing chips on the same order as the pitch between neighboring pads. Inaddition, in order to allow chips in multi-chip modules (MCMs) that usePxC to be replaced (as needed), a remateable assembly technique isdesirable.

One purely mechanical assembly technique that provides highly accurateand remateable mechanical coupling of chips in an MCM includes balls andpits. In this assembly technique, adjacent chips in the MCM are alignedby placing the balls into collocated pits on surfaces of the chips.

An existing MCM that includes balls and pits is shown in FIG. 1. In thisMCM, island chips (such as processors) are placed face-down, and abridge chip is placed face-up. Note that the chips receive signals,power and ground from the package substrate through C4 solder and/orcopper pillars as level-one interconnects, and that the bridge chipcommunicates with the chips using PxC.

Typically, there are mechanical clearances, h1, h2 and h3, between thecomponents in this existing MCM. However, given these mechanicalclearances, it is difficult to remateably support the bridge chip inFIG. 1 while maintaining the alignment of the balls and pits. Forexample, because of mechanical clearance h3, if the balls are merelyplaced into the pits (which provides remateable mechanical coupling)there is no normal or restoring force to oppose the force of gravity onthe bridge chip. Consequently, the bridge chip will fall away from theisland chips, which results in misalignment and poorer PxC.

Hence, what is needed is an MCM without the above-described problems.

SUMMARY

One embodiment of the present disclosure provides a multi-chip module(MCM) that includes: a first substrate having a first surface, a secondsubstrate having a second surface that faces the first surface, andpositive features. Note that the first substrate includes first negativefeatures disposed on the first surface, where a given first negativefeature is recessed below the first surface and has an opening, definedby an edge. Moreover, at least a subset of the first negative featuresincludes a first magnetic material. Furthermore, the positive featuresmechanically couple the first substrate and the second substrate bymating with associated first negative features, where the mechanicalcoupling is facilitated by the first magnetic material.

In some embodiments, the positive features are disposed on the secondsurface, where a given first positive feature protrudes above the secondsurface. Furthermore, the positive features may include a secondmagnetic material. For example, the second magnetic material may includea soft magnetic material at room temperature.

Note that the first magnetic material may include a hard magneticmaterial at room temperature. Furthermore, the first material mayundergo a ferromagnetic to anti-ferromagnetic phase transition above agiven temperature, thereby facilitating disassembly of the MCM.

Additionally, a remainder of the first negative features may include thesecond magnetic material.

In some embodiments, the second substrate includes second negativefeatures disposed on the second surface, where a given second negativefeature is recessed below the second surface and has an opening, definedby an edge. Moreover, the given second negative feature may beassociated with the given first negative feature, thereby defining agiven pair of negative features.

Note that at least a subset of the second negative features may includethe first magnetic material. Furthermore, at least the subset of thesecond negative features may have a mirror spatial configurationrelative to at least the subset of the first negative features so that,in the given pair of negative features, only one of the given firstnegative feature and the given second negative feature includes thefirst magnetic material. Additionally, a remainder of the secondnegative features may include the second magnetic material.

In some embodiments, the positive features may be separate from thesecond substrate. For example, the positive features may includemicrospheres, and a given microsphere may, at least in part, mate withthe given first negative feature and the given second negative feature.Note that the positive features may include the second magneticmaterial.

In some embodiments, the second substrate includes the second magneticmaterial, which provides a flux-return path to at least the subset ofthe first negative features which include the first magnetic material.

Another embodiment provides an electronic device that includes the MCM.

Another embodiment provides a method for fabricating the first substratefor use in the MCM. During the method, the first negative features aredefined on the first surface of the first substrate, where the givenfirst negative feature is recessed below the first surface and has anopening, defined by an edge. Then, a seed layer is deposited in at leastthe subset of the first negative features. Next, the first magneticmaterial is deposited on the seed layer in at least the subset of thefirst negative features. Note that the seed layer defines an orientationof a magnetic axis of the first magnetic material. Moreover, the firstsubstrate is configured to mechanically couple in the MCM to the secondsubstrate having the second surface that faces the first surface viapositive features in the MCM that are configured to mate with the firstnegative features. Furthermore, the mechanical coupling is facilitatedby the first magnetic material.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram illustrating an existing multi-chip module(MCM).

FIG. 2A is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 2B is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 2C is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 2D is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 3 is a block diagram illustrating a substrate for use in an MCM inaccordance with an embodiment of the present disclosure.

FIG. 4 is a block diagram illustrating a substrate for use in an MCM inaccordance with an embodiment of the present disclosure.

FIG. 5A is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 5B is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 5C is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 6 is a flow chart illustrating a process for fabricating magneticstructures in accordance with an embodiment of the present disclosure.

FIG. 7 is a flow chart illustrating a process for fabricating magneticstructures in accordance with an embodiment of the present disclosure.

FIG. 8 is a flow chart illustrating a process for fabricating a firstsubstrate for use in an MCM in accordance with an embodiment of thepresent disclosure.

FIG. 9 is a block diagram illustrating an electronic device inaccordance with an embodiment of the present disclosure.

Note that like reference numerals refer to corresponding partsthroughout the drawings. Moreover, multiple instances of the same typeof part are designated by a common prefix separated from an instancenumber by a dash.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the disclosure, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present disclosure is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

Embodiments of a multi-chip module (MCM), an electronic device thatincludes the MCM, and a technique for fabricating a substrate in the MCMare described. This MCM includes at least two substrates that areremateably mechanically coupled by positive features (such asmicrospheres or balls) and negative features (such as pits) on facingsurfaces of the substrates. These positive and negative features matewith each other. In particular, a positive feature may mate with a givenpair of negative features, which includes negative features on each ofthe substrates. Furthermore, at least one of the negative features inthe given pair may include a hard magnetic material, and the positivefeature and the other negative feature in the given pair may include asoft magnetic material that provide a flux-return path to the hardmagnetic material. In this way, the hard material may facilitate theremateable mechanical coupling of the substrates.

In addition to remateably coupling the substrates, thismechanical-coupling technique may align the substrates, and may becompatible with high-volume manufacturing. In particular, the positiveand negative features may be fabricated on the surfaces usingsemiconductor-process techniques, and the hard magnetic material mayenhance yield during assembly by preventing the positive features fromfalling out of the negative features. Furthermore, because themechanical coupling is remateable, the MCM can be reworked, which mayreduce the extent of chip testing prior to assembly of the MCM.Therefore, this mechanical-coupling technique may reduce the cost of theMCM.

We now describe embodiments of an MCM. In the discussion that follows,static magnetic forces provided by a magnetic ball-and-pit structure areused to mechanically couple and maintain the relative positions ofcomponents in the MCM, both during and after assembly. Furthermore, bychanging the magnetic properties and the geometry of the magneticmaterials in the magnetic ball-and-pit structure, the resulting magneticforces can be selected so that components in the MCM, such as islandchips and/or bridge chips, are remateable.

FIG. 2A presents a block diagram illustrating a side view of an MCM 200.This MCM includes: a substrate 210-1 having a surface 212-1, a substrate210-2 having a surface 212-2 that faces surface 212-1, and positivefeatures 214 (such as microspheres or balls). Note that substrates 210include negative features 216 (such as pits) disposed on surfaces 212,where a given negative feature (such as negative feature 216-1) isrecessed below a given surface (such as surface 212-1) and has anopening, defined by an edge. Moreover, a given negative feature onsurface 212-1 (such as negative feature 216-1) may be associated with agiven negative feature on surface 212-2 (such as negative feature216-2), thereby defining a given pair 218 of negative features.

As shown below in FIG. 3, at least a subset of negative features 216 mayinclude a first magnetic material in layers 220, and a remainder ofnegative features 216 may include a second magnetic material in layers222. Furthermore, positive features 214 mechanically couple substrates210 by mating with associated negative features 216 (for example, inpair 218). This mechanical coupling may be facilitated by staticmagnetic forces associated with the first magnetic material in layers220.

Note that the first magnetic material may include a hard magneticmaterial, such as a permanent magnet that has a large remnantmagnetization in zero external magnetic field. This hard magneticmaterial may include a ferromagnetic material or a ferromagneticmaterial at room temperature, such as: an alloy of: iron, cobalt,iron-oxide, manganese, manganese-bismuth, nickel, antimony, nickel-iron,samarium-cobalt, neodymium, barium-titanate, and/orstrontium-barium-titanate. In some embodiments, the Curie temperature ofthe hard magnetic material is greater than 500 C so that assemblyprocesses, such as bonding and solder reflow, do not affect the magneticproperties.

Furthermore, the second magnetic material may include a soft magneticmaterial, i.e., a material with a large magnetic susceptibility (orpermeability), a low coercivity and a negligible remnant magnetizationin zero external magnetic field. This soft magnetic material may includea paramagnetic material, a ferromagnetic material or a ferromagneticmaterial at room temperature, such as a nickel-iron alloy. Additionally,positive features 214 may include the second magnetic material. Thesecond magnetic material in these components may provide flux-returnpath to the first magnetic material in layers 220 (as illustrated by thedashed field lines 224). This flux-return path may be enhanced by theorientation of hard magnetic axes in the second magnetic material. Forexample, positive features 214 may each have a hard axis that isapproximately perpendicular to surfaces 212 in the plane of FIG. 2A.Alternatively, positive features 214 may have four-fold symmetry, sothat each positive feature (such as positive feature 214-1) has a hardaxis every 90° in the plane of FIG. 2A. This configuration may ensurethat a hard axis of each of the positive features 214 is within 45° ofthe perpendicular to surfaces 212.

In some embodiments, the shape of positive features 214, at least inpart, determines the orientation of the hard axis in these components.For example, the positive features 214 may each have an ellipticalshape.

During assembly of MCM 200, positive features 214 may be placed intonegative features 216 on individual dies or substrates 210 while theyare still in wafer form. The static magnetic forces may help holdpositive features 214 in place when the chips are flipped, aligned withand mechanically coupled to chips having collocated negative features216. Note that this mechanical-coupling technique is compatible with theMCM configuration shown in FIG. 1, and may provide a restoring forcethat holds the bridge chips in place.

A variety of configurations may be used to increase the strength of thestatic magnetic forces. For example, as shown in FIG. 2B, which presentsa block diagram illustrating a side view of an MCM 240, either or bothof substrates 210 may include additional layers 250 that include thesecond magnetic material. These additional layers may provide aflux-return path to layers 220 that include the first magnetic material(e.g., field lines 224 may go from layer 220-1, through positive feature214-1, layer 222-1, layer 250-2, layer 250-1 and back to layer 220-1).

In some embodiments, both negative features in a given pair of negativefeatures (such as pair 218) include the first magnetic material. This isshown in FIG. 2C, which presents a block diagram illustrating a sideview of an MCM 260. In this MCM, the orientation (north-south) of themagnetization of the first magnetic material in the negative features onsubstrate 210-1 may be opposite to the orientation (north-south) of themagnetization of the first magnetic material in the negative features onsubstrate 210-2.

Note that the first magnetic material may extend beyond negativefeatures 216 in optional layers 270, thereby providing traces thatincrease the strength of the static magnetic force between substrates210. Additionally, in some embodiments positive features 214 include thefirst magnetic material. For example, positive features 214 may includea demagnetized ferrimagnetic material or ferromagnetic material (such asiron), which may be magnetized by the external magnetic field providedby the first magnetic material in layers 220. Alternatively, thepositive features 214 may be magnetized by an external magnetic fieldduring the assembly of MCM 260. However, in some embodiments positivefeatures 214 include a ferrimagnetic material or a ferromagneticmaterial that is magnetized prior to the assembly of MCM 260. In theseembodiments, pick-and-place equipment may be used to keep positivefeatures 214 from mechanically coupling to each other during theassembly process.

While the preceding examples illustrated positive features 214 as beingseparate from substrates 210, in some embodiments positive features aredisposed on one of substrates 210. This is shown in FIG. 2D, whichpresents a block diagram illustrating a side view of an MCM 280. Inparticular, positive features 290 are disposed on and protrude abovesurface 212-2. Alternatively, positive features 290 may be disposed onsurface 212-1 or on both surfaces 212. As shown in FIG. 2D, in someembodiments positive features 290 include the second magnetic material.Alternatively, positive features 290 may include the first magneticmaterial.

Note that positive features 214 (FIGS. 2A-2C) and/or 290 may have a widevariety of shapes, including: a sphere, a hemisphere, a top hat or apyramid shape. Similarly, negative features 216 in these embodiments caninclude: a pit, a trench, an inverse pyramidal shape and/or a groove.

As noted previously, in some embodiments only a subset of negativefeatures 216 may include the first magnetic material. This configurationmay prevent the occurrence of a ‘conflict’ between the magneticorientations of the magnetization in layers 220 in a given pair (such aswhen a north pole faces a north pole or a south pole faces a south pole,which results in a repulsive force between substrates 210). In addition,if the negative features in the subset have an asymmetric configuration,then by rotating a chip the mirror-image configuration may be obtained.This is shown in FIG. 3, which presents a block diagram illustrating atop view of a substrate 210-1. Because the negative features onsubstrate 210 in subset 310 that include the first magnetic materialinterdigit with their mirror image (and with the mirror image ofnegative features 216 in remainder 312 that include the second magneticmaterial), this configuration ensures that only one of the negativefeatures in the given pair includes the first magnetic material. It alsoallows chips with a single north-south orientation of the first magneticmaterial to be used in MCMs, which can reduce the cost and complexity ofthese systems.

In some embodiments, the negative features are elongated, which canimprove alignment and assembly. This is shown in FIG. 4, which presentsa block diagram illustrating substrate 210-1. This substrate includeselongated negative features 410, which provide a degree of mechanicalfreedom that positive features (as illustrated by the dashed circle) canmove in. Thus, these elongated negative features can accommodaterelative motion between substrates in an MCM (for example, due tothermal expansion, shock and/or vibration), and the first magneticmaterial in layers 220 (which is deposited in the middle of theelongated negative features 410) may provide a corrective or a restoringforce that subsequently realigns the substrates.

Note that the use of this mechanical-coupling technique, and thepresence of the static magnetic force(s) that help hold the componentstogether, may facilitate alternate configurations for MCMs than theconfiguration shown in FIG. 1. Several exemplary configurations areshown in FIGS. 5A-5C.

In FIG. 5A, which presents a block diagram illustrating an MCM 500, thecarrier package is coupled to the chips by C4 solder balls, and thecavity for the bridge chip in the carrier package is face down. In someembodiments, there is an insulating underfill around the C4 solderballs, for example, if a flip-chip assembly process is used.

In FIG. 5B, which presents a block diagram illustrating an MCM 520, thecarrier package is coupled to the chips using wire bonding, and thecavity for the bridge chip in the carrier package is face up.Alternatively, FIG. 5C, which presents a block diagram illustrating anMCM 540, shows a carrier package with a face-down cavity and wirebonding.

Note that in the MCMs shown in FIGS. 5A-5C, the magnetic force(s)associated with the magnetic material may facilitate self-alignment andassembly. For example, components may be placed proximate to each otherusing a pick-and-place tool, and the magnetic force(s) may then alignand mechanically couple these components.

As noted previously, the magnetic components in the precedingembodiments may provide remateable mechanical coupling. For example, themagnetic force(s) may be strong enough to hold the components together,but may be overcome (if needed) using a pick-and-place tool.Alternatively, the first material may undergo a ferromagnetic toanti-ferromagnetic phase transition above a given temperature, which mayallow an MCM to be disassembled by applying heat (for example, using ahair dryer or a heater in a substrate). In other embodiments, thecomponents in the preceding embodiments may be released by demagnetizingthe first magnetic material using an external magnetic field (forexample, using AC demagnetization). Furthermore, cold welding oradhesion of components may be reduced or eliminated (therebyfacilitating remateable mechanical coupling) by roughening or depositingcoatings on surfaces of components, such as positive features 214 (FIGS.2A-2D) and negative features 216 (FIGS. 2A-2D).

While the preceding embodiments used static magnetic force(s) tofacilitate the mechanical coupling, in some embodiments dynamic ortime-varying magnetic forces are used. For example, negative features216 (FIGS. 2A-2D) may be coated with an electrically conductive materialand/or a soft magnetic material, and positive features 214 (FIGS. 2A-2D)may include electromagnets. These electromagnets may be selectivelyturned on or off, which allows the magnetic-coupling force to bedynamically switched on or off. In the off-state, a component (such asthe bridge chip) could be ‘released’ from associated island chips. Thiscapability facilitates the physical disengagement or disconnection ofone (or more) chips from a bank of chips. For example, a processor canbe disconnected from a main processor array by physically breaking aninterconnection. This is quite different from an electronic or opticalswitch because it involves a physical break of the circuit. In additionto allowing MCMs to be reworked, this capability may offer degrees offreedom in system security and control.

In order to use magnetic balls and pits for alignment and to provideremateable mechanical coupling, the resulting magnetic force typicallyneeds to be strong enough to support the weight of the balls and/or thebridge chip. In the following calculation, the configuration shown inFIG. 2C is used as an example, with permanent magnetic spheres and aferromagnetic material in the pits on substrates 210 (FIG. 2C). Notethat for a silicon bridge chip, which is 5 mm long, 6 mm wide and 150 μmthick, the weight is 0.01 g (where g is the acceleration of gravity atthe Earth's surface). Similarly, for a neodimium permanent magneticsphere, with a radius of 90 μm, the weight per ball is 0.018 mg.Therefore, the weight of the silicon bridge chip and four spheres isaround 0.01 g.

The magnetic force is estimated by assuming that the entire surface of amagnetic sphere is in contact with the iron ferromagnetic material inthe pits, and that the force is proportional to

$\frac{\mu_{o}B_{m}^{2}A_{m}}{2},$

where μ_(o) is the permeability of free space, B_(m) is the magneticflux density of the permanent magnetic sphere, and A_(m) is theeffective contact area in the pits. With four neodymium permanentmagnetic spheres, and assuming closed-flux paths, the total attractiveforce is 0.19 g, which is nineteen times larger than the force ofgravity on the four balls and the bridge chip.

Note that the magnetic fields associated with the first magneticmaterial and/or the second magnetic material are expected to havenegligible impact on circuits on substrates 210 (FIGS. 2A-2D) becausethe magnetic fields are static.

We now describe embodiments of processes. FIG. 6 presents a flow chartillustrating a process 600 for fabricating magnetic structures (such asmagnetic pits), which may be performed aftercomplementary-metal-oxide-semiconductor (CMOS) processing. During thisprocess, openings for the pits may be patterned over keepout regions ona wafer using lithography. Then, using photoresist as a soft mask, thedielectric stacks may be removed by plasma etching processing (operation610). Subsequently, a layer of silicon nitride may be deposited acrossthe wafer; windows over the pit regions may be processed using alithography process; and, after nitride removal (for example, by plasmaetching), the pits may be formed using wet etching (for example, in atetramethyl ammonium hydroxide solution or a potassium hydroxidesolution) (operation 612).

After pit etching to a defined depth by a timed etch, the siliconnitride layer is removed, and lithography is used to pattern openingsover the pits. Next, a seed layer (such as titanium) is deposited in thepits (operation 614).

Moreover, a magnetic material (such as the first magnetic material) isdeposited on the seed layer (operation 616). Note that the magneticmaterial may be deposited using a variety of processing techniques,including: sputtering, electrolytic plating and/or electroless plating.For example, a conductive (metal) magnetic layer may be fabricated byelectroless plating of gold and nickel-phosphate.

In an exemplary embodiment, aluminum (such as aluminum-silicon oraluminum-copper) is deposited in the pits on top of a titanium seedlayer. After the photoresist has been stripped, the wafers are putthrough an electroless nickel-gold process. Note that a thick layer ofelectroless nickel may provide a ferromagnetic material in the pits.Also note that electroless nickel-gold is a common under-bumpmetallization used to layer input/output pads before bumping.Consequently, this choice of material may serve multiple functions.

However, if a different under-bump-metallization material is used,process 600 may be modified so that the aluminum deposition and theelectroless nickel-gold process occur before the nitride hard mask usedfor pit etching is removed. Note that process 600 may be simplified byeliminating the metal fill in regions where the pits are to be etched.Furthermore, a guard ring around the pit areas may limit dielectricdelamination outside of the pits during the pitting process.

FIG. 7 presents a flow chart illustrating a process 700 for fabricatingmagnetic structures, which may be performed prior to CMOS processing.During this process, silicon nitride is deposited on the bare siliconwafer and is patterned to serve as a hard mask. Then, pits are etched inthe wafer by wet-etching as described previously (operation 710).Subsequently, the nitride hard mask is then removed and the CMOSdielectric stack is fabricated around the pits. On completion of CMOSfabrication, another lithography operation is used to pattern windowsabove the pits, which are then cleaned out using dry etch processing(operation 712). Next, a third lithography and liftoff process is usedto deposit a seed layer in the pits (operation 714), followed bydeposition of the magnetic material (for example, electrolessnickel-gold) (operation 716).

A more general process is shown in FIG. 8, which presents a flow chartillustrating a process 800 for fabricating a first substrate for use inan MCM, such as MCM 200 (FIG. 2A). During the method, first negativefeatures are defined on a first surface of a first substrate (operation810), where a given first negative feature is recessed below the firstsurface and has an opening, defined by an edge. Then, a seed layer isdeposited in at least a subset of the first negative features (operation812). Next, a first magnetic material is deposited on the seed layer inat least the subset of the first negative features (operation 814). Notethat the seed layer may define an orientation of a magnetic axis of thefirst magnetic material. Moreover, the first substrate is configured tomechanically couple in the MCM to a second substrate having a secondsurface that faces the first surface via positive features in the MCMthat are configured to mate with the first negative features.Furthermore, the mechanical coupling is facilitated by the firstmagnetic material.

In some embodiments, at least some of negative features 216 (FIGS.2A-2D) are filled to increase the magnetic flux linkage or coupling. Forexample, in a typical ball-and-pit assembly, the contact area between aball and a pit may be limited to the line of intersection between thetwo components. Increasing this contact area may increase themagnetic-flux linkage between the ball and the pit. To achieve this,after the magnetic material is deposited in FIGS. 6-8, a small amount ofa suitable low-modulus material may be deposited inside of the pits.This low-modulus material may be thick enough to be compressed duringassembly yet thin enough so that it does not limit the magnetic flux. Inan exemplary embodiment, the low-modulus material may be a magneticpolymer. Furthermore, the low-modulus material may be deposited using:binding equipment, a stencil printing process, and/or micro-fabrication.

The magnetic structures in the preceding embodiments of the MCMs mayfacilitate wafer-level ball placement and wafer handling. In particular,after processing and wafer dicing, the pits may be populated withmagnetic balls. Strong magnetic forces between the magnetic balls andthe pit metal may hold the balls in place at the time of pick, transferand during wafer flipping.

A variety of techniques may be used to populate the pits with balls in abatch process. For example, a patterned magnetic plate may be used. Inthis technique, a unique assembly fixture is used to populate the ballsinto the pits on a wafer or a single chip. This fixture may include apatterned magnetic plate with raised bosses at locations correspondingto the layout of the pits on the wafer or chip. During assembly, thepatterned side of the magnetic plate may be placed face-to-face with thepitted side of the wafer/chip. Then, magnetic balls having appropriatedimension may be sprinkled onto the magnetic plate. These balls mayattach to the magnetic plate at the raised boss locations. Next, themagnetic plate may be aligned with a pitted wafer in a face-to-facemanner, and the two substrates may be brought together and agitated.Note that balls at locations corresponding to pits may remain in place,while the mechanical agitation may cause extraneous balls to roll off.

Alternatively, inductive coupling may be used to populate the balls inthe pits. In particular, a fixture wafer with micro-inductors or coilspatterned at locations corresponding to pits on the corresponding wafermay be used to position the balls. Note that inductive coupling througha substrate is usually strong. Consequently, when balls are sprinkledonto the inductor wafer, the strong inductive coupling may align theballs and hold them in place. Then, the inductor wafer may be alignedwith and brought in contact with a matching pitted wafer. Once the ballsare held by the magnetic material in the pits, the micro-inductors maybe switched off and the fixture wafer may be removed.

In some embodiments of processes 600 (FIG. 6), 700 (FIG. 7) and/or 800there are additional or fewer operations. For example, the orientationof the magnetic axis of the first magnetic material in process 800 maybe determined, at least in part, by an applied external magnetic field.Moreover, the order of the operations may be changed, and/or two or moreoperations may be combined into a single operation.

One or more of the preceding embodiments of the MCM may be included in asystem and/or an electronic device. This is shown in FIG. 9, whichpresents a block diagram illustrating an electronic device 900. Thiselectronic device includes MCM 910.

In general, an MCM may include an array of chip modules (CMs) orsingle-chip modules (SCMs), and a given SCM may include at least onesubstrate, such as a semiconductor die. Note that an MCM is sometimesreferred to as a ‘macro-chip.’ Furthermore, the substrate maycommunicate with other substrates, CMs and/or SCMs in the MCM usingproximity communication of electromagnetically coupled signals (which isreferred to as ‘electromagnetic proximity communication’). For example,the proximity communication may include: communication of capacitivelycoupled signals (electrical proximity communication') and/orcommunication of optical signals (such as ‘optical proximitycommunication’). In some embodiments, the electromagnetic proximitycommunication includes inductively coupled signals and/or conductivelycoupled signals.

Furthermore, embodiments of the MCM may be used in a variety ofapplications, including: VLSI circuits, communication systems (such asin wavelength division multiplexing), storage area networks, datacenters, networks (such as local area networks), and/or computer systems(such as multiple-core processor computer systems). For example, the MCMmay be included in a backplane that is coupled to multiple processorblades, or the MCM may couple different types of components (such asprocessors, memory, input/output devices, and/or peripheral devices). Insome embodiments, the MCM performs the functions of: a switch, a hub, abridge, and/or a router.

Note that electronic device 900 may include, but is not limited to: aserver, a laptop computer, a communication device or system, a personalcomputer, a work station, a mainframe computer, a blade, an enterprisecomputer, a data center, a portable-computing device, a supercomputer, anetwork-attached-storage (NAS) system, a storage-area-network (SAN)system, and/or another electronic computing device. Moreover, note thata given computer system may be at one location or may be distributedover multiple, geographically dispersed locations.

MCMs and substrates in FIGS. 2A-5C and/or electronic device 900 mayinclude fewer components or additional components. For example, negativefeatures 216 (FIGS. 2A-2D) may be defined in layers that are depositedon surfaces 212 (FIGS. 2A-2D), and these negative features may berecessed below a surface of the top layer deposited on substrates 210(FIGS. 2A-2D). Similarly, positive features 290 (FIG. 2D) may protrudeabove a local surface, which may be surface 212-2 (FIG. 2D) or a surfaceof a top layer deposited on substrate 212-2 (FIG. 2D). Thus, in thepreceding embodiments a surface of a substrate should be understood toinclude a surface of a layer deposited on the substrate or a surface ofthe substrate itself.

Furthermore, although these embodiments are illustrated as having anumber of discrete items, these MCMs and electronic devices are intendedto be functional descriptions of the various features that may bepresent rather than structural schematics of the embodiments describedherein. Consequently, in these embodiments two or more components may becombined into a single component, and/or a position of one or morecomponents may be changed.

Note that positive features 214 (FIGS. 2A-2C), positive features 290(FIG. 2D) and/or negative features 216 (FIGS. 2A-2C) may be definedusing an additive process (i.e., a material-deposition) and/or asubtractive process (i.e., a material-removal). For example, the processmay include: plating, sputtering, isotropic etching, anisotropicetching, a photolithographic technique and/or a direct-write technique.Additionally, these features may be fabricated using a wide variety ofmaterials, including: a semiconductor, metal, glass, sapphire, and/orsilicon dioxide.

The foregoing descriptions of embodiments of the present disclosure havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present disclosure tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present disclosure. The scope ofthe present disclosure is defined by the appended claims.

1. A multi-chip module (MCM), comprising: a first substrate having afirst surface, wherein the first substrate includes first negativefeatures disposed on the first surface, wherein a given first negativefeature is recessed below the first surface and has an opening, definedby an edge, and wherein at least a subset of the first negative featuresincludes a first magnetic material; a second substrate having a secondsurface that faces the first surface; and positive features that areconfigured to mechanically couple the first substrate and the secondsubstrate by mating with associated first negative features, wherein themechanical coupling is facilitated by the first magnetic material. 2.The MCM of claim 1, wherein the positive features are disposed on thesecond surface; and wherein a given first positive feature protrudesabove the second surface.
 3. The MCM of claim 2, wherein the positivefeatures include a second magnetic material.
 4. The MCM of claim 3,wherein the second magnetic material includes a soft magnetic materialat room temperature.
 5. The MCM of claim 1, wherein the first magneticmaterial includes a hard magnetic material at room temperature.
 6. TheMCM of claim 5, wherein the first material undergoes a ferromagnetic toanti-ferromagnetic phase transition above a given temperature, therebyfacilitating disassembly of the MCM.
 7. The MCM of claim 1, wherein aremainder of the first negative features include a second magneticmaterial.
 8. The MCM of claim 7, wherein the second magnetic materialincludes a soft magnetic material at room temperature.
 9. The MCM ofclaim 1, wherein the second substrate includes second negative featuresdisposed on the second surface, wherein a given second negative featureis recessed below the second surface and has an opening, defined by anedge; and wherein the given second negative feature is associated withthe given first negative feature, thereby defining a given pair ofnegative features.
 10. The MCM of claim 9, wherein the positive featuresinclude microspheres; and wherein a given microsphere at least in partmates with the given first negative feature and the given secondnegative feature.
 11. The MCM of claim 10, wherein the positive featuresinclude a second magnetic material.
 12. The MCM of claim 11, wherein thesecond magnetic material includes a soft magnetic material at roomtemperature.
 13. The MCM of claim 9, wherein at least a subset of thesecond negative features include the first magnetic material.
 14. TheMCM of claim 13, wherein the first magnetic material includes a hardmagnetic material at room temperature.
 15. The MCM of claim 13, whereinat least the subset of the second negative features has a mirror spatialconfiguration relative to at least the subset of the first negativefeatures so that, in a given pair of negative features, only one of thegiven first negative feature and the given second negative featureincludes the first magnetic material.
 16. The MCM of claim 13, wherein aremainder of the second negative features include a second magneticmaterial.
 17. The MCM of claim 16, wherein the second magnetic materialincludes a soft magnetic material at room temperature.
 18. The MCM ofclaim 1, wherein the second substrate includes a second magneticmaterial that provides a flux-return path to at least the subset of thefirst negative features which include the first magnetic material. 19.An electronic device, comprising an MCM, wherein the MCM includes: afirst substrate having a first surface, wherein the first substrateincludes first negative features disposed on the first surface, whereina given first negative feature is recessed below the first surface andhas an opening, defined by an edge, and wherein at least a subset of thefirst negative features includes a first magnetic material; a secondsubstrate having a second surface that faces the first surface; andpositive features that are configured to mechanically couple the firstsubstrate and the second substrate by mating with associated firstnegative features, wherein the mechanical coupling is facilitated by thefirst magnetic material.
 20. A method for fabricating a first substratefor use in an MCM, comprising: defining first negative features on afirst surface of a first substrate, wherein a given first negativefeature is recessed below the first surface and has an opening, definedby an edge; depositing a seed layer in at least a subset of the firstnegative features; and depositing a first magnetic material on the seedlayer in at least the subset of the first negative features, wherein theseed layer defines an orientation of a magnetic axis of the firstmagnetic material; wherein the first substrate is configured tomechanically couple in the MCM to a second substrate having a secondsurface that faces the first surface via positive features in the MCMthat are configured to mate with the first negative features; andwherein the mechanical coupling is facilitated by the first magneticmaterial.